1. Field of the Invention
The present invention relates generally to an optical attenuators and processes for manufacturing optical attenuators, and, more particularly, to an optical attenuator and a process of manufacturing optical attenuators able to attenuate incident light of an optical fiber by varying a refractive index of a core layer of the optical fiber.
2. Description of the Related Art
Generally, when processing an optical communication signal carried by an optical transmission network, an optical signal exhibiting some degree of strength within a light receiving range of an optical receiver module should be received. If the strength of the optical signal exceeds the range of capacity for the light receiver, an error is likely to occur in the optical receiver module that will cause a serious problem in an operating lifetime. To solve theses disadvantages, an optical attenuator has been used.
Optical attenuators may be classified as either a plug-in type or an in-line type, as are discussed in greater detail in conjunction with attenuators illustrated by FIGS. 1 and 2 of this application. Examples of earlier efforts in the art are exemplified by way of illustration, by the Ruggedized Grated Optical Fiber of Jack E. Goodman, et alii, U.S. Pat. No. 4,593,969; the Optical Waveguide Embedded Transverse Spatial Mode Discrimination Filter of William H. Glenn, et alii, U.S. Pat. No. 5,048,913; the Optical Fiber Laser Or Amplifier Including High Reflectivity Gratings of Stephen G. Grubb, U.S. Pat. No. 5,323,404; and the Optical Fiber Package of William M. MacDonald, et alii, U.S. Pat. No. 5,367,589; and the techniques for creating gratings mentioned in Bragg Grating Made In Optical Waveguide of Elias Snitzer, et alii, U.S. Pat. No. 5,351,321, in Method Of Fabricating Bragg Gratings Using A Silica Glass Phase Grating Mask And Mask Used By Same of Kenneth O. Hill, et alii, U.S. Pat. No. 5,367,588, the Incubated Bragg Gratings In Waveguides of Kevin C. Byron, et alii, U.S. Pat. No. 5,574,810, the Optical Waveguide With Diffraction Grating And Method Of Forming The Same, of Hans Bruesselbach, U.S. Pat. No. 5,604,829, and the Method Of Detecting And/Or Measuring Physical Magnitudes using a Distribution Sensor of And/oe Tardy, U.S. Pat. No. 5,684,297.
Both plug-in and an in-line types of attenuators often seek to attenuate the strength of incident light traveling between two adjoining lengths of optical fiber by inserting a thin film optical filter into a ferrule or a sleeve. In order to attenuate the incident light by reflecting or absorbing the optical signal, the thin film optical filter is coated to a multilayer structure by using various kinds of metal elements and finally processes its both surfaces by non-reflection coating so as to maintain a non-reflectance of 99.8% or more.
With conventional designs of plug-in type of optical attenuators, however, because it has been difficult to process the thin film filter to obtain a non-reflective coating that provides non-reflectance of 99.8% or more, any optical signal that is reflected in a very high speed optical transmission network of 2.5 Giga-bits per second or more backwardly enters the interior of the optical fiber. Consequently, an error in the optical signal is likely to occur. Moreover, since the thin film filter bearing the thin film coating and the non-reflective coating may easily become separated due to temperature and humidity, the characteristics of the optical signal may vary according to the wavelength of the optical signals. Furthermore, since the optical fiber is cut at an angle of eight degrees and the thin film filter is fixed between the ferrules in order to attenuate the incident light of the optical fiber, even though no contact is made by external components and the thin film filter during coupling of the optical connector, additional components must be used in order to connect the optical adaptor to the optical distribution box. These additional optical components increase the cost of a product and make it exceedingly difficult to pack the optical connectors in a dense array in the optical distribution box.
Additionally, with conventional optical in-line attenuators, since it is difficult to obtain a thin film filter with the non-reflection coating providing a non-reflectance of 99.8% or more, the optical signal reflected in the very high speed optical transmission network of 2.5 Gbps or more is reflected backwardly into the interior of the optical fiber and causes errors in the optical signal. Moreover, due to separation of the thin film filter bearing the thin film and non-reflection coating due to temperature and humidity, the characteristics of the optical signal tend to vary according to wavelength. Furthermore, since the middle portion of the optical cable is cut and the thin film filter inserted between the adjoining end surfaces in order to attenuate the incident light of the optical fiber, the tensile strength of the optical cable is reduced and it is difficult to process and manage the presence of any additional optical cable within the optical distribution box.